Cadmium vapor discharge lamp containing a europium activated phosphor

ABSTRACT

A cadmium vapor discharge lamp containing a bivalent europium activated luminescent substance.

United States Patent inventors Mijndert Koedam;

Willem Lambertus Wanmaker, Emmasingel, Eindhoven, Netherlands Appi. No.756,933

Filed Sept. 3, 1968 Patented Jan. 12, 1971 Assignee U. S. PhilipsCorporation,

New York, N.Y.

a corporation of Delaware, by mesne assignments Sept. 2, 1967Netherlands Priority CADMIUM VAPOR DISCHARGE LAMP CONTAINING A EUROPIUMACTIVATED PHOSPHOR 10 Claims, 2 Drawing Figs.

Primary Examiner-John Kominski Assistant Examiner-David OReilly AttornevFrank R. Trifari ABSTRACT: A cadmium vapor discharge lamp containing abivalent europium activated luminescent substance.

PATENTEU m1 2 ml sum 1 or 2 INVENTORS MIJNDERT KOEDAM WILLEM L .WANMAKERBY W I-X$ AGEN CAllMlUli/l VAPOR DISQHARGE LAMP CONTAlNllNG A EUROPIUMAUHVATED PHOSPHOR For many purposes radiation sources are employed whichcomprise the combination of a gas discharge tube and a luminescentscreen containing one or more substances adapted to be excited by theradiation produced in the gas discharge and to emit radiation of higherwavelength as a result thereof. The luminescent substances may belocated inside or outside the discharge space. It is generally known touse a combination of a low-pressure mercury vapor discharge tube and aluminescent screen which converts a large portion of the ultravioletradiation produced in the mercury-vapor discharge into visibleradiation. The luminescent substances are formed by highly differentcompounds. These compounds usually consist of crystal latticescontaining one or more activators. The invention also relates to aradiation consisting of the combination of a gas discharge tube and aluminescent screen. However, in contrast to the aforesaid knownradiation sources, the gas discharge tube contains cadmium vapor insteadof mercury vapor.

The use of cadmium vapor in a discharge lamp is known. However, thislamp has not yet been employed in practice, since it has not been knownwhich luminescent substances satisfy the most important specificrequirements applying to the combination of these substances with acadmium-vapor discharge, that is to say an appropriate excitationspectrum and a practically useful quantum efiiciency.

In the known radiation sources consisting of the combination of alow-pressure mercury-vapor discharge tube and a luminescent screen thesame considerations are, of course, due with respect to an appropriateexcitation spectrum and a satisfactory quantum efficiency. As statedabove, quite different compounds are known to form luminescent materialsuitable for being combined with a mercury-vapor discharge tube and itapplies to said compounds that they satisfy the above-mentionedrequirements to a greater or lesser extent. These substances are excitedfor an important part by the mercury spectrum lines of a wavelength of185.0 and 253.7 nm. The spectrum of a cadmium-vapor discharge, which isvery much similar to the spectrum of a mercury-vapor discharge, exhibitsthe corresponding spectrum lines at higher wavelengths, that is to say:228.8 and 326.1 nm. From this difference it will be seen that substancessuitable for being combined with a low-pressure mercury-vapor dischargeneed not be suitable for the combination with a low-pressurecadmiumvapor discharge. Very little is stated in literature on thissubject.

The useof the combination of a low-pressure cadmiumvapor discharge andan appropriate luminescent substance has, for purely theoreticalreasons, the advantage that the energy efficiency of the combination maybe higher than that of the combination of a low-pressure mercury-vapordischarge and an appropriate luminescent substance, exactly because theexciting radiation of the cadmium-vapor spectrum has higher wavelengths.lt is then supposed as a matter of course, for the quantum efficiency ofthe luminescent materials employed to have the same value.

A radiation source according to the invention comprises a low-pressurecadmium-vapor discharge tube and is characterized in that it comprises aluminescent screen containing a substance activated by bivalenteuropium, which converts at least part of the radiation emitted by thedischarge in the cadmium vapor into radiation of higher wavelength.

The invention is based on elaborate investigations made both with knownluminescent substances activated by bivalent europium and luminescentsubstances hitherto not described and activated by bivalent europium. Itappears that all these substances have an excitation spectrum whichsatisfactorily matches the emission spectrum of a low-pressurecadmiumvapor discharge. Although the conversation efficiency of variousluminescent substances activated by bivalent europium may be different,it has been found to be possible to manufacture practically usableradiation sources according to the invention by means of many of thesesubstances. A few part'icu' larly suitable combinations will bedescribed in detail hereinafter. t

it is known that in a low-pressure mercury-vapor discharge tube, at avapor pressure of about 10 u, the highest energy efficiency of theconversation of the electric energy supplied to the discharge tube intoradiation of a wavelength of X850 and 253.7 nm. respectively isobtained. This optimum vapor pres sure is obtained at a temperature ofthe discharge tube of about 40 C. Since this temperature does notdeviate too much from normal room temperature, the use of low-pressuremercury-vapor discharge tubes has met with the known boom.

The optimum cadmium-vapor pressure in a low-pressure cadmium-vapordischarge tube is also about 10 a. This optimum vapor pressure isobtained at a temperature of the discharge tube of about 270 C. For thepractical use in a radiation source it is therefore necessary, if a highenergy efficiency of the conversion of the electric energy fed to thedischarge tube into radiation of wavelengths of 228.8 and 326.1 nm.,respectively is desired, to take steps to attain and maintain this walltemperature, inter alia by restricting the thermal losses. The situationis therefore similar to that of the known sodium vapor discharge lampsand the precautions taken with these lamps for restricting thermallosses may also be applied to a low-pressure cadmium-vapor dischargetube. It

is therefore preferred to surround a low-pressure cadmiumvapor dischargetube for use in a radiation source according to the invention by anouter bulb which restricts the loss of heat of the discharge tube to theambience. The space between the low-pressure cadmium-vapor dischargetube and the outer bulb is preferably exhausted so that the loss of heatis very slight. As with the sodium vapor discharge lamp the loss of heatmay be further reduced by applying a layer of high infrared reflectivepower to the inner side of the outer bulb. This layer has, of course, tobe transparent to the radiation to be emitted by the source. Suitablematerial for such a layer is, for example, conductive tin oxide, orconductive indium oxide. Also thin gold layers provide satisfactoryresults.

The luminescent material employed in a radiation source according to theinvention may be provided at different places with respect to thecadmium-vapor discharge. The simplest method consists in the applicationto the inner side of the wall of the cadmiumvapor discharge tube. Thisapplication has the advantage that this wall need not be pervious to theshortwave ultraviolet radiation produced by the cadmium discharge. Adisadvantage is, however, that the luminescent substance has to beresistant to the chemical attack by cadmium. The situation is differentfrom that of a low-pressure mercury-vapor discharge lamp, since asstated above the tempera-= ture of the discharge tube has to exhibit theaforesaid high value in order to attain a high energy efficiency. If atthis temperature the luminescent substance cannot withstand thedischarge, operational conditions of the discharge tube have to beaccepted, in which the temperature is lower and hence the efficiency issmaller. Apart from the chemical resistance the luminescent substancehas to emit sufficient radiation of long wavelength at the temperaturerequired for a high efficiency. With most luminescent substances thequantity of emitted radiation diminishes, as is known, fairly stronglyat temperatures exceeding C. The substances activated by bivalenteuropium employable in a radiation source according to the invention aresuch with respect to chemical resistance and temperature-dependence ofthe conversion that they can be applied to the inner side of the wall ofthe discharge space.

if the low-pressure cadmium-vapor discharge tube is surrounded for theaforesaid reasons by an outer bulb, the luminescent substance may alsobe applied to the outer side of the cadmium-vapor discharge tube or tothe inner side of the outer bulb. To these two places it applies thatthe wall of the cadmium-vapor discharge tube has to be satisfactorilypervious to the short-wave ultraviolet radiation and has to be made, forexample, of quarts. if the luminescent substance is applied to the outerside of the cadmium-vapor discharge tube, the

aforesaid requirement applies that also at the high temperature thesubstance should be sufficiently emissive. The chemical resistance tothe hot cadmium vapor does no longer play any part.

Since the inner side of the outer bulb has a much lower temperature thanthe wall of the cadmium-vapor discharge tube, it is more advantageous toapply the luminescent substance to said side. If it is desired, asstated above, to restrict the loss of heat by applying a heat-reflectinglayer to the inner side of the outer bulb, the adhesion of theluminescent substance to this outer bulb already covered by theheat-reflective layer may sometimes give rise to difficulties. inaccordance with the respective requirements, the nature of theluminescent substance and the manufacturing process one of theabove-mentioned three places for the luminescent substance will bepreferred.

A further possibility consists, of course, in applying the luminescentsubstance to the outer side of the outer bulb or to a separatereflector. In this case the outer bulb has to be made of a materialwhich is satisfactorily pervious to short-wave ultraviolet radiation.

Which luminescent substance or which mixture of substances is employedin a radiation source according to the invention is dependent upon theintended use of the radiation source. If the radiation source isintended for use in apparatus for reproducing documents employinglightsensitive paper, a substance will be chosen which has a strongemission in the blue and/or long-wave ultraviolet part of the spectrum.An appropriate known substance is, for example, calcium fluorideactivated by bivalent europium. It is described for this substance thatwhen excited by ultraviolet radiation of a wavelength of 365.0 or 253.7nm. from a mercury-vapor discharge it exhibits an emission having amaximum at about 420 nm. It has been found that the same emission alsooccurs upon excitation by the radiation from a low-pressurecadmium-vapor discharge.

A further known substance activated by bivalent europium is bariumsilicate of the basic lattice composition of 2Ba0.1S iO This substancehas a maximum emission at about 510 nm., both with an excitation by aradiation of a wavelength of 365.0 and 253.7 nm. respectively (which isknown) cadmiumvapor with an excitation by a radiation from alow-pressure cadmium-vapor discharge. The last-mentioned substance maybe used in a source of radiation according to the invention in tendedfor illumination purposes.

Luminescent substances activated by bivalent europium which are not yetknown will be described in detail hereinafter with reference to a fewexamples of manufacturev Some of these substances are particularlysuitable for use in radiation sources intended for document-reproducingapparatus, others are more suitable for use in radiation sources forillumination purposes.

In a radiation source according to the invention there is preferablyused a luminescent substance satisfying the formula:

wherein A designates at least one of the elements strontium and calciumand og gms olsgz.

The properties of these luminescent substances are substantiallyindependent of the ratio between the elements designated by A. A maytherefore be strontium or calcium or a mixture of these two elements.

The condition: 1.90 x +y z +p q 2.05 indicates that the substance shouldapproximately have the pyrophosphate composition. As is known, it issometimes more advantageous to base the manufacture of a complex latticewith a plurality of elements on a mixture in which the quantities of theconstituents do not completely satisfy the stoechiometric ratio in thepyrophosphate. A small excess quantity of one or more of the basiccomponents often provides a higher yield of the formation reaction. Thereaction product then contains a small excess quantity of one or more ofthe basic components. This reaction product satisfies the aforesaidcondition of x y z p q, but the luminescent substance proper presumablysatisfies accurately the stoechiometric formula of the pyrophosphate.Since it has been found that the residues of basic substances leftusually have only a negligible influence on the luminescence, it is notalways necessary to remove them.

The elements barium and magnesium, as will be evident from theconditions for y and z, may be absent. The spectral distribution of theemission spectrum is practically not affected by the presence of barium;more than 1.2 molecules of barium produces, however, a reduction of theconversion efficiency of the ultraviolet radiation and is thereforeundesirable.

The quantity of magnesium may be chosen larger, i.e. 1.6 moleculeswithout the radiation conversion efficiency being reduced excessively.The use of a quantity of magnesium exceeding one-fourth of the quantityof calcium and strontium together gives rise to the production of anemission spectrum having two peaks, i.e. one at about 420 nm. and one atabout 391 nm. The intensities of the emitted radiations at these twowavelengths are correlated so that with an increase of the quantity ofmagnesium the intensity of the peak at 391 nm. constantly increases andthe intensity of the peak at 420 nm. constantly decreases until at aquantity of magnesium oxide of 1.2 mol. practically no radiation is anylonger emitted at 420 nm. The radiation at 391 nm. is then, however,very strong.

The condition that y +2 should at the most be equal to 1.6 is requiredsince otherwise when choosing the maximum quantity of magnesium and themaximum quantity of barium no calcium and/or strontium could be presentin the luminescent substance. One of these two elements is, however,always required and the value ofx should be at least 0.15.

The quantity of bivalent europium oxide may be varied between theaforesaid limits, but it is preferable chosen between 0.01 and 0.04. Inthis region lies the highest radiation efficiency.

From the condition: 0 5 q E 0.15 it appears that the luminescentsubstance need not invariably contain manganese. If manganese ispresent, an emission is also obtained in the red part of the spectrum.By activating it by bivalent europium and manganese this substance has astrong emission in the blue part and in the red part of the spectrum.The radiation source is therefore suitable for illumination purposes,when a satisfactorily color rendition is wanted. The fact that thesubstances activated by bivalent europium and manganese emit onlyslightly in the green and yellow parts of the spectrum is of littleimportance since the cadmium discharge itself exhibits fairly strongemission lines in this very part. These emission lines lie at 467.8 nm.,480.0 nm. and 508.0 nm. At these wave lengths the luminescent substancessatisfying the above-mentioned formula do practically not emitradiation. As compared with low-pressure mercury-vapor discharge lampsthe com bination of these substances with a low-pressure cadmiumvapordischarge lamp is considerably more favorable. The correspondingemission lines of the low-pressure mercury-vapor discharge spectrum liepartly in the very region in which also most luminescent substancesemployed emit a fairly strong radiation. Therefore more radiation may beemitted with these wavelengths than is desirable for a satisfactorycolor rendition.

This disadvantage is not involved in a radiation source according to theinvention.

A luminescent substance particularly suitable for a combination with alow-pressure cadmium-vapor discharge tube to form a radiation source forillumination purposes satisfies the formulas:

Substances satisfying this formula and said conditions provide a highdegree of efficiency in conjection with a satisfactory color rendition,particularly when Apart from the above-mentioned advantages of theluminescent materials suitable for use in a radiation source accordingto the invention the substances have low sensitivity to oxidation. Thisis very important in the manufacture of the radiation source, since thesubstances are often exposed for a short time to heating in air at afairlyhigh temperature, for example 600 C. Such heating may be required,when an organic binder is used which has to be removed afterwards byheating.

The invention will now be described with reference to a drawing and to afew examples of manufacture of luminescent substances suitable for usein a radiation source according to the invention.

In the drawing FIG. 1 shows schematically an embodiment of a radiationsource according to the invention.

FIG. 2 of the drawing is a graph in which the intensity of theluminescent radiation is plotted in arbitrary units on the ordinate andthe wavelength on the abscissa in manometers. The curves of the graphillustrate the variation of the intensity for the following examples.The maximum for each curve is fixed at 100.

Referring to FIG. 1, reference numeral 1 designates a lowpressurecadmium-vapor discharge tube having the form of a U. Two and threedesignate the cathodes. In the embodiment shown the wall 4 of thisdischarge tube is made of vitreous quartz. The discharge tube 1 issurrounded by an outer bulb 5, for example, of hard glass-6 designates alayer of luminescent material the layer 6 is applied to the innerside ofthe outer bulb 5 by coating said innerside with a suspension containingthe luminescent material and an organic binder e.g. nitrocellulose thecoated bulb is then dried, after which the organic binder is removed byheating the discharge tube 1 contains cadmium vapor and a rare gas or arare-gas mixture for starting the discharge and for improving theoutput. Such a gas or gas mixture, for example, neon or neon plus asmall supply of argon, is used as is known also in mercury-vapordischarge lamps and sodium lamps.

EXAMPLES OF PRODUCTION.-EXAMPLE 1 A mixture of 6.904 gs. of SrHPO g. Of(NHOZ HPO4, 0.070 g. Of Eu2O is made and thoroughly mixed in a morserand put into a crucible. The crucible with its contents is thenintroduced into a furnace, in which it is heated at a temperature of1,200 C. for two hours, while a stream ofair is passed over. Afterheating the reaction product is refined in a morser and sieved through asieve of 220p. mesh. The material passed through the sieve is againheated at a temperature of l,200 C. for two hours in a furnace, throughwhich air is passed. After cooling of the crucible with its contents toroom temperature, the reaction product is ground to fineness and sieved.It is then ready for use.

The composition of the reaction produce satisfies the formula:

1-88SIO.1 P205I0.02 EHO.

Upon excitation by ultraviolet radiation from a low-pressurecadmium-vapor discharge the luminescent material emits a radiation whosespectral distribution is illustrated by curve 1 in FIG. 2 of thedrawing.

Heating may be carried out in a neutral or slightly reducing atmosphere,for example, in a mixture of nitrogen and 0.1 to 8 percent by volume ofhydrogen, instead of being carried out in air. This often provides ahigher radiation output of the luminescent material. Presumably, thismay be accounted for by a more complete conversation of the Eu O intoEu0. This conversion is obtained, however, also by heating in air. Thismay be accounted for by the fact that apparently the europium ispreferably incorporated in the crystal lattice in the bivalent form,since the ion radii of strontium and bivalent europium are practicallythe same.

The reducing atmosphere may, if desired, also be obtained by placing inthe furnace at the side of the crucible containing the mixture ofcrucible containing finely divided carbon and by passing over air or adifferent oxygen-containing gas. The oxygen and the carbon form carbonmonoxide which constitutes the reducing atmosphere above the reactionmixture.

EXAMPLE 2 A mixture of 4.333 gs. of SrHPO 2.177 gs. of CaHPO 0.053 g. of(NH HPO and 0.070 g. of E1120.

is made and treated in the manner described in example 1. The resultantluminescent material satisfies the formula:

1, 18Sr0.0.80Ca0. 1P O 10.02Eu0.

Upon excitation by ultraviolet radiation from a low-pressurecadmium-vapor discharge the luminescent material emits a radiation whosespectral distribution is also illustrated by the curve 1 of FIG. 2 ofthe drawing.

EXAMPLE 3 A mixture of 5.388 gs. of CaHPO 0.053 g. of (NH HPO Of E1130?is made and treated completely in the same manner described in example1.

The resultant luminescent material satisfies the formula:

EXAMPLE 4 A mixture of 5.802 gs. of SrHPO 1.867 gs. of BaHPO g. Of(NH4)2HPO4, g. Of Ellg03 is made and treated completely in the samemanner as described in example 1.

The resultant luminescent material satisfies the formula:

1.58Sr0.0.40Ba0.1P O :0.02Eu0.

Upon excitation by ultraviolet radiation from a low-pressurecadmium-vapor discharge the luminescent material emits a radiation whosespectral distribution is also illustrated by the curve 1 of EXAMPLE 5 Amixture of 5.802 gs. 0f SrHPO 1.099 gs. of MgNH PO 0.053 g. of (NH HPO0.070 g. of Eu O3 is made and treated completely in the same manner asdescribed in example I.

The resultant luminescent material satisfies the formula:

1.58SrO.0.40MgO.1P O 10.02EuO.

Upon excitation by ultraviolet radiation from a low-pressurecadmiumwapor discharge the luminescent material emits a radiation whosespectral distribution is also illustrated by the curve I of FIG. 2 ofthe drawing.

EXAMPLE 6 A mixture of 6.427 gs. of Sr-HPO; 0.660 g. of (NH HPO g- OfE1130;

is made and treated completely in the same manner as described inexample I.

The resultant luminescent material satisfies the formula:

1.75SrO.1P O :0.15EuO.

Upon excitation by ultraviolet radiation from a low-pressurecadmium-vapor discharge the luminescent material emits a radiation whosespectral distribution is also illustrated by the curve 1 of FIG. 2 ofthe drawing.

EXAMPLE 7 A mixture of 5.068 gs. of SrHPO 0.544 g. of OaHPO 0.549 g. ofMgNH PO 0.933 g. of BaHPO 0.053 g. of (NH HPO 0.070 g. of Eu O is madeand treated completely in the same manner as described in example 1.

The resultant luminescent material satisfies the formula:

1.38Sr0.020C.O.0.20MgO.0.20BaO.1P O

0.02EuO.

Upon excitation by ultraviolet radiation from a low-pressurecadmium-vapor discharge the luminescent material emits a radiation whosespectral distribution is also illustrated by the curve 1 of FIG. 2 ofthe drawing.

EXAMPLE 8 A mixture of 5.802 gs. of SrHPO 0.933 g. of BaHPO 0.544 g. ofCaHPO g. Of (NH4)3HPO4, g. of EUZO3 is made and treated completely inthe same manner as described in example 1.

The resultant luminescent material satisfies the formula:

1.58SrO.0;BaO.0.20CaO.P O :0.02Eu0.

Upon excitation by ultraviolet radiation from a low-pressurecadmium-vapor discharge the luminescent material emits a radiation whosespectral distribution is also illustrated by the curve 1 of FIG. 2 ofthe drawing.

EXAMPLE 9 A mixture of 2.864 gs. of SrHPO 3.296 gs. of MgNH PO 0.052 gof (NH HPO g. Of B11203 is made and treated completely in the samemanner as described in example 1.

The resultant luminescent material satisfies the formula:

Upon excitation by ultraviolet radiation from a low-pressurecadmium-vapor discharge the luminescent material emits a radiation whosespectral distribution is illustrated by the curve 2 of FIG. 2 ofthedrawing.

EXAMPLE 10 A mixture of 5.116 gs. of Cal-IP0 0.053 g. of (NH HPO 0.336g. of MnNH PO 0.070 g. of Eu O is made and treated completelyinthe samemanner as described in example 1. t

The resultant luminescent material satisfies the formula:

1.88Ca0.1P O :0.02EuO.0.10MnO.

EXAMPLE 11 A mixture of 6.904 gs. of SrHPO 0.053 g. of (NH HPO g. OfMDNH4PO4, g. Of B11203 is made and treated completely in the describedin example 1.

The resultant luminescent material satisfies the formula:

1.88Sr0.1P O :0.02EuO.0.10MnO.

Upon excitation by ultraviolet radiation from a low-pressurecadmium-vapor discharge the luminescent material emits a radiation whosespectral distribution is illustrated by the curve 4 in FIG. 2 of thedrawing.

In FIG. 2 of the drawing the vertical lines indicate the places of themaxima of the radiation in the visible part of the spectrum emanatingfrom cadmium-vapor discharge itself. It is apparent that these maximaare located in the very valley of the curves 3 and 4.

We claim:

1. A cadmium-vapor discharge lamp comprising a glass envelo e, cadmiumvapor, a means to roduce an electric disc arge in said envelope and abivalent europium activated phosphor positioned to receive radiationresulting from an electric discharge in said cadmium vapor said phosphorpredominantly responsive to radiation at the wavelengths of 228.8 nm.and 326.1 nm.

2. The lamp of claim 1 wherein the phosphor is located on the inner sideof the glass envelope.

3. The lamp of claim 1 wherein the glass envelope is surrounded by anouter bulb.

4. The lamp of claim 3 wherein the phosphor is located on the innersideof the outer bulb.

5. The lamp of claim 3 wherein the phosphor is located on the outer sideof the glass envelope.

6. The lamp of claim 3 wherein the space between the glass enveloped andthe outer bulb is exhausted.

7. The lamp of claim 3 wherein the inner side of the outer bulb isprovided with a layer which strongly reflects infrared rays but isprevious to visible or ultraviolet radiation.

8. The lamp of claim 1 wherein the phosphor satisfies the formula samemanner as wherein A designates at least one of the elements strontiumand calcium and 9. The lamp of claim 1 wherein the phosphor satisfiesthe 10. The lamp of claim 1 wherein the phosphor satisfies the formulaformula UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3 555 335 (BE 2702) Dated ilgmlgnz 12 191] Inventor(s) MIJNDERT KOEDAMET AL It is certified that error appears in the above-identified patentand that said Letters Patent are hereby corrected as shown below:

' Column 2, line 74,

Column 4, line 5,

Column 5, line 38,

line 44,

Column 8, line 56,

Signed and sealed (SEAL) Attest:

EDJARD M.FIETGHER,JR. Attesting Officer "quarts" should read quartzafter I 1.90" insert "q" insert "enveloped" should read envelope this30th day of November 1971.

ROBERT GOTTSCHALK Acting Commissioner of Pat

2. The lamp of claim 1 wherein the phosphor is located on the inner side of the glass envelope.
 3. The lamp of claim 1 wherein the glass envelope is surrounded by an outer bulb.
 4. The lamp of claim 3 wherein the phosphor is located on the innerside of the outer bulb.
 5. The lamp of claim 3 wherein the phosphor is located on the outer side of the glass envelope.
 6. The lamp of claim 3 wherein the space between the glass enveloped and the outer bulb is exhausted.
 7. The lamp of claim 3 wherein the inner side of the outer bulb is provided with a layer which strongly reflects infrared rays but is previous to visible or ultraviolet radiation.
 8. The lamp of claim 1 wherein the phosphor satisfies the formula
 9. The lamp of claim 1 wherein the phosphor satisfies the formula
 10. The lamp of claim 1 wherein the phosphor satisfies the formula 